Diaryl carbonates have been used for the production of polycarbonates. The production of diaryl carbonates can proceed by production of phosgene and subsequent reaction of phosgene with monophenols. However, phosgene used for the production of diaryl carbonates may contain impurities such as carbon tetrachloride that results in the formation of organic chlorides as impurities in the produced diaryl carbonates, particularly diphenyl carbonate. Diphenyl carbonates containing high levels of organic chlorides are unsuitable for use in polycarbonate synthesis as they adversely impact the polymerization reaction and may also result in adverse color. Thus there is a strong incentive to use phosgene having low levels of organic chloride compounds for the synthesis of diaryl carbonates in general and diphenyl carbonate in particular.
In one method for producing phosgene, carbon monoxide is reacted with chlorine in the presence of a carbon-comprising catalyst such as activated carbon or silicon carbide. The reaction is strongly exothermic and is usually performed in a reactor such as a multi-tubular reactor that has been designed similarly to conventional shell and tube heat exchangers.
A carbon tetrachloride by-product can result from the phosgene reaction and can be present in an amount of 50 to 300 parts per million (ppm) by volume or higher. Carbon tetrachloride can be formed in the phosgene reaction via multiple reaction routes, one of which involves the direct chlorination of catalyst carbon. The presence of high levels of carbon tetrachloride in phosgene as an impurity can be disadvantageous in the production of diaryl carbonates. Presence of high amounts of carbon tetrachloride leads to an increase of organic impurities in the diaryl carbonate which might cause a reduction of the catalytic activity in the polymerization reaction as well as discoloration issues in the final polycarbonate resin. According to U.S. Pat. No. 8,044,226, 1 ppm of chlorinated impurities is sufficient to inhibit the polymerization reaction, whereas less than 1 ppb is preferably in order to synthesize an uncolored polycarbonate with perfect transparency.
Current processes for the purification of diphenol carbonate are mostly based on a cascade of distillation columns. For example, U.S. Pat. No. 5,734,004 discloses a purification method based on distillation in which diphenol carbonate is removed in vapor phase from a side-draw. U.S. Pat. No. 7,812,189 discloses that the purification method is able to produce a high-purity diphenol carbonate with less than 1 ppb of chlorides. WO2012/076532 discloses the purification of diphenol carbonate from chlorides, metals and other heavy contaminants by using a non-porous membrane or nanofiltration membrane with a pore size up to 10 nm. EP0722931A1 discloses a method to prepare a high-purity diphenyl carbonate free of chlorinated impurities by distillation in the presence of a basic substance.
However, phosgene purification to remove carbon tetrachloride can be difficult and is a significant part of capital investment and operating costs of any phosgene plant due to the costly material of construction of the purification equipment, the need for large enclosures to house said equipment, and further because the process is very energy intensive. On a global basis, the amount of byproduct carbon tetrachloride produced in commercial phosgene manufacturing annually can be as much as 2 million kilograms based on phosgene production of about 4 billion kilograms.
A method to produce diaryl carbonates without the need for a separate phosgene purification process is therefore desirable.